A light-emitting diode (LED) is known for its high energy efficiency and is introduced to various fields of use. The theory for a light-emitting diode (LED) to emit light is that when a forward voltage power is applied to a p-n junction, the electrons are driven from the n-type semiconductor and the holes are driven from the p-type semiconductor, and these carriers are combined in the active layer to emit light. The efficiency of an LED depends on the combination rate of the electrons from the n-type semiconductor and the holes from the p-type semiconductor. However, due to the poor current spreading, especially in the p-type semiconductor, the efficiency is reduced. An electrode with an extending part such as a finger-type electrode is commonly used to improve the poor current spreading. In addition, a transparent conductive layer is disposed between the finger-type electrode and the p-type semiconductor as an ohmic contact layer to improve the current spreading.
FIG. 1 is a schematic structure diagram of a conventional light-emitting diode without the finger-type p-electrode. FIG. 1A is the top view, and FIG. 1B is the cross-sectional view along the line W-W′. As shown in FIG. 1, a conventional light-emitting device 10 comprises a substrate 100, a light-emitting stack 110, a transparent conductive layer 104, and two electrodes 105, 106. The light-emitting stack 110 comprises a first conductivity type semiconductor layer 101, a second conductivity type semiconductor layer 103, and an active layer 102 between the first conductivity type semiconductor layer 101 and the second conductivity type semiconductor layer 103. For example, the first conductivity type semiconductor layer 101 is n-type, and the second conductivity type semiconductor layer 103 is p-type. The first electrode 105 is electrically connected to the first conductivity type semiconductor layer 101, and the second electrode 106 is electrically connected to the transparent conductive layer 104. When an external power source is supplied to the light-emitting device 10 from the two electrodes 105, 106, the condition of electrons flow (the reverse of the current/holes flow) from the first electrode 105 to the second electrode 106 is shown as the arrow lines indicate. As the second electrode 106 is not an electrode with an extending part, a current crowding phenomenon occurs and there is almost no electrons flow in the area A which is not under the second electrode 106. This phenomenon results in a non-uniform light emission and low luminous efficiency.
To solve the problem described above, a conventional light-emitting diode with the finger-type p-electrode shown in FIG. 2 is provided. FIG. 2A is the top view, and FIG. 2B is the cross-sectional view along the line W-W′. The conventional light-emitting device 20 is substantially the same as the light-emitting device 10, except that the second electrode 206 is a finger-type electrode, which comprises an extending part 206a in addition to the main part 206b. Other elements, such as a substrate 200, a light-emitting stack 210, a first conductivity type semiconductor layer 201, an active layer 202, a second conductivity type semiconductor layer 203, and a transparent conductive layer 204, are the same as those shown in FIG. 1, and are not discussed again.
As illustrated in FIG. 2, the extending part 206a is introduced to solve the current crowding problem. When an external power source is supplied to the light-emitting device 20 from the two electrodes 205, 206, the condition of electrons flow (the reverse of the current/holes flow) from the first electrode 205 to the second electrode 206 is shown as the arrow lines indicate. Because the second electrode 206 is a finger-type electrode, the current/holes is(are) spread by the extending part 206a, and the deficiency which there is almost no electrons flow in the area A in FIG. 1 is improved. However, as shown in FIG. 2, it is observed that electrons flow in that area A is a majority part, and only a few of electrons flow through the area B in FIG. 2.